offscreen: Render GPUI views into images on iOS and Android

Cargo.toml

@@ -48,6 +48,9 @@ calendar = []
 deeplink = []
 microphone = []
 media_session = []
+# Render GPUI windows into images (`gpui_mobile::offscreen`). Needs the vendored
+# `gpui-pre-wgpu` (see `[patch.crates-io]`), which exposes its headless renderer.
+offscreen = ["gpui_wgpu/offscreen", "dep:image"]
 
 [lib]
 name = "gpui_mobile"
@@ -61,6 +64,8 @@ log          = { version = "0.4", features = ["kv_unstable_serde", "serde"] }
 parking_lot  = "0.12"
 futures      = "0.3"
 raw-window-handle = "0.6"
+# Same major version as `gpui-pre-wgpu`, whose `RgbaImage` `offscreen` returns.
+image        = { version = "0.25", default-features = false, optional = true }
 
 # ── GPUI core crate (provides Platform trait, event types, geometry, etc.) ───
 gpui = { package = "gpui-pre", version = "=0.3.8", default-features = false }
@@ -139,3 +144,10 @@ strip         = true
 [profile.dev]
 opt-level = 0
 debug     = true
+
+# `gpui-pre-wgpu` 0.3.8 with its headless renderer behind an `offscreen` feature
+# instead of `test-support` / `bench-support`, and a constructor that takes the
+# wgpu instance (iOS needs Metal). Applications that enable `offscreen` must
+# patch it the same way.
+[patch.crates-io]
+gpui-pre-wgpu = { path = "vendor/gpui-pre-wgpu" }

README.md

@@ -89,6 +89,35 @@ cd android/gradle && ./gradlew assembleDebug
 | Android (arm64) | ✅ | API 26+ | Vulkan (preferred), GL ES 3.0 |
 | Android (armv7/x86_64) | ⚠️ Untested | API 26+ | Vulkan / GL ES |
 
+## Rendering views to images
+
+The `offscreen` feature adds `gpui_mobile::offscreen::OffscreenWindow`, a GPUI
+window drawn into a texture instead of onto the screen, for turning a view into
+an image (a share card, a long screenshot):
+
+```rust
+// Open with the view to capture, then render from an async task
+// (`render` drives the window's own frame callbacks, which borrow the `App`).
+let window = OffscreenWindow::open(size(px(390.), px(844.)), 3.0, WindowAppearance::Light, cx, |window, cx| {
+    cx.new(|cx| MyView::new(window, cx))
+})?;
+window.layout(size(px(390.), px(844.)))?; // lay out without reading pixels back, e.g. to measure content
+let image = window.render(size(px(390.), content_height))?; // image::RgbaImage
+window.close(cx);
+```
+
+It needs `gpui-pre-wgpu`'s headless renderer, which upstream only builds for
+tests and benchmarks and which picks a Vulkan/GL device that iOS lacks. This
+repository vendors `gpui-pre-wgpu` 0.3.8 with that renderer behind an
+`offscreen` feature and a constructor that takes the wgpu instance
+(`vendor/gpui-pre-wgpu`). An application enabling `offscreen` must patch it the
+same way:
+
+```toml
+[patch.crates-io]
+gpui-pre-wgpu = { git = "https://github.com/longbridge/gpui-mobile" }
+```
+
 ## Screenshots
 
 Running on a Motorola Edge 50 Pro (Android, Vulkan):

src/android/platform.rs

@@ -1148,6 +1148,10 @@ impl Platform for AndroidPlatform {
         _handle: AnyWindowHandle,
         _options: WindowParams,
     ) -> anyhow::Result<Box<dyn PlatformWindow>> {
+        #[cfg(feature = "offscreen")]
+        if let Some(window) = crate::offscreen::take_pending(&_options) {
+            return Ok(window);
+        }
         // On Android the native window is created by the system via
         // MainEvent::InitWindow, not by the application.  If a window
         // already exists we wrap it in an AndroidPlatformWindow and hand

src/ios/platform.rs

@@ -161,6 +161,10 @@ impl Platform for IosPlatform {
         handle: AnyWindowHandle,
         options: WindowParams,
     ) -> anyhow::Result<Box<dyn PlatformWindow>> {
+        #[cfg(feature = "offscreen")]
+        if let Some(window) = crate::offscreen::take_pending(&options) {
+            return Ok(window);
+        }
         let window = Box::new(IosWindow::new(handle, options)?);
         // Register the window with FFI layer so Objective-C can access it for rendering
         window.register_with_ffi();

src/lib.rs

@@ -79,6 +79,8 @@ pub(crate) mod frame_demand;
 #[cfg_attr(not(target_os = "android"), allow(dead_code))]
 pub(crate) mod frame_pacer;
 pub mod momentum;
+#[cfg(all(feature = "offscreen", any(target_os = "ios", target_os = "android")))]
+pub mod offscreen;
 pub mod packages;
 pub mod platform_view;
 pub mod target_platform;